Dry reagent test devices using compositions containing chromogenic indicator materials incorporated in porous or absorbent matrices have become the systems of choice to determine the amount of various analytes in aqueous fluids. Even though such test devices excel for use in field or on-site testing situations, they can likewise be advantageously used in laboratory settings. Such systems, commonly called reagent strips or "dip-and-read" test devices, can be made to give semi-quantitative and to some extent quantitative results when the user understands the limitations of the test methodology and the sample makeup. One serious problem commonly encountered by test strip developers and users is the presence of interfering colored components or materials in the test sample fluid or solution. These interfering colored components may be the result of the introduction of extraneous substances, such as naturally occurring colored materials often found in samples of ground water, or may be the result of the purposeful addition of dyestuffs to identify or delineate the particular fluid being analyzed, such as in the identification of a particular type of coolant fluid used in association with internal combustion engines.